A shotgun mic is a UNI-directional condensor which records ONE direction, only. All shotgun mics use a standard directional capsule — usually a supercardioid — the hope is that the tighter directionality (at high frequencies), will reduces the ambient noise. A "diaphragm" mic, as you called it, has a typically wider pickup pattern, which gets more room ambience, and the less desirable for filmmaking.